Under the influence of hormones, the mammary gland undergoes marked developmental changes to express its differentiated function during lactation. This system serves as a good model to elucidate the molecular mechanisms by which hormones regulate developmental and tissue-specific gene expression in mammalian cells. Previous studies show that the lactogenic hormones stimulate transcription of milk protein genes by inducing specific transcription factors. During the course of our study, we have discovered and cloned a cDNA, which encodes a transcription-activating factor for the murine selenocysteine tRNA gene and named this mStaf. The selenocysteine tRNA is essential for the biosynthesis of selenoprotein as a donor of selenocysteine. By transfection experiments we demonstrated that mStaf plays a critical role in regulation of selenocysteine tRNA gene transcription. Our data also showed that the level of mStaf in the mammary gland increases markedly in parallel with the increase of selenocysteine tRNA during pregnancy and lactation when the gland undergoes growth and development. We also found that the lactogenic hormones increase the level of mStaf transcript and protein through the MAPK pathway and enhance the selenocysteine tRNA levels in the mammary gland. Our present study revealed the structure of the mStaf gene. This gene is approx. 35 kb long and split into 16 exons. All exon-intron junction sequences conform to the GT/AG rule. The transcription start site is located 83 bp upstream of the initiation codon. This gene is mapped to mouse chromosome 7. Transfection experiments showed that the 5'-flanking region (-1894 to +37) of the mStaf gene drives transcription in mouse cells. Deletion and mutation studies suggested that Sp1 sites play an important role for the basal promoter activity. In addition to these studies we have isolated one of the major selenoproteins, iodothyronine deiodinase from the mammary gland. This enzyme plays a key role in the formation of active thyroid hormones during the development of mammary gland during pregnancy. We characterized its gene structure including the 5'-upstream region and examined its regulatory properties in response to lactogenic hormones.